This invention relates to a multi-piece solid golf ball comprising at least four layers, a solid core, an enclosure layer, an intermediate layer, and a cover and having high resilience, good flight performance and a very soft pleasant feel.
Many solid golf balls such as two-piece golf balls are known in the art. As compared with wound golf balls, solid golf balls have the advantage of an increased total flight distance on both driver and iron shots, because of a so-called straight line trajectory and a low spin rate due to their structure, which allows for a long run. On the other hand, solid golf balls are more difficult to control than the wound golf balls in that they do not stop short on the green because of low spin receptivity on iron shots.
Like flight distance, a pleasant feel when hit is essential for golf balls. The absence of a pleasant feel represents a substantial loss of commodity value. As compared with the solid golf balls, wound golf balls have the structural characteristics ensuring a soft and pleasant feel.
For two-piece solid golf balls consisting of a core and a cover, attempts have been made to soften the ball structure in order to accomplish a soft feel upon impact. However, such attempts fail to fully meet the demand. By providing an intermediate layer between the core and the cover, three-piece solid golf balls were obtained. Although many proposals were made, it was still difficult to provide a golf ball having both the flight distance of two-piece solid golf balls and the feel of wound golf balls.
Recently, multi-piece solid golf balls having at least four layers were proposed (see JP-A 9-266959, 10-127818, and 10-127819). One solid golf ball proposed is of the four-layer structure in which a three-layer structure solid core consisting of an internal layer, an intermediate layer and an outer layer is enclosed with a cover. The ball is improved in hitting feel and control by providing a difference in hardness between the respective layers.
However, if the difference in hardness between two adjacent layers is reduced, little improvement in hitting feel is achieved. If the difference in hardness between two adjacent layers is significant or if the selection of the thickness or material of the adjacent layers is inappropriate, the deflection or deformation upon impact becomes largely different between the adjacent layers. This causes an energy loss at the interface therebetween and thus detracts from resilience, resulting in a reduced flight distance. The durability of the ball against consecutive strikes is also lost. This tendency becomes outstanding particularly when two adjacent layers are formed of different materials which cannot be strongly joined.
An object of the invention is to provide a multi-piece solid golf ball comprising a solid core, an enclosure layer, an intermediate layer, and a cover which has a very soft feel while maintaining the flight performance of solid golf balls.
The invention is directed to a multi-piece solid golf ball of at least four-layer structure comprising a solid core, an enclosure layer surrounding the core, an intermediate layer of at least one layer surrounding the enclosure layer, and a cover of at least one layer surrounding the intermediate layer. The inventor has found that by properly selecting the hardness, thickness and material of the respective layers, and the change rates of deflection under load of spherical bodies before and after the provision of a layer, the multi-piece solid golf ball can be improved in resilience and control and provided with a very soft, pleasant feel.
In a first aspect, the invention provides a multi-piece solid golf ball comprising a solid core, an enclosure layer around the core, an intermediate layer of at least one layer around the enclosure layer, and a cover of at least one layer around the intermediate layer, wherein the enclosure layer and the intermediate layer each have a Shore D hardness of 10 to 50, and the Shore D hardness of the enclosure layer is not greater than the Shore D hardness of the intermediate layer which is not greater than the Shore D hardness of the cover. Provided that the solid core has a deflection A (mm) under an applied load of 100 kg, a first spherical body consisting of the solid core and the enclosure layer has a deflection B (mm) under an applied load of 100 kg, a second spherical body consisting of the solid core, the enclosure layer and the intermediate layer has a deflection C (mm) under an applied load of 100 kg, and the ball has a deflection D (mm) under an applied load of 100 kg, these defection values satisfy the relationship that A is from 2.5 to 7.0 mm, 0.85xe2x89xa6B/Axe2x89xa61.15, 0.85xe2x89xa6C/Bxe2x89xa61.15, and 0.7xe2x89xa6D/Cxe2x89xa61.0.
In a second aspect, the invention provides a multi-piece solid golf ball comprising a solid core, an enclosure layer of at least one layer around the core, an intermediate layer around the enclosure layer, and a cover of at least one layer around the intermediate layer, wherein the enclosure layer and the intermediate layer each have a Shore D hardness of 10 to 50, the Shore D hardness of the enclosure layer is not greater than the Shore D hardness of the intermediate layer which is not greater than the Shore D hardness of the cover, and the cover is formed of a cover stock comprising a thermoplastic resin as a base component and an inorganic filler. Provided that the solid core has a deflection A (mm) under an applied load of 100 kg, a first spherical body consisting of the solid core and the enclosure layer has a deflection B (mm) under an applied load of 100 kg, a second spherical body consisting of the solid core, the enclosure layer and the intermediate layer has a deflection C (mm) under an applied load of 100 kg, and the ball has a deflection D (mm) under an applied load of 100 kg, these defection values satisfy the relationship that A is from 2.5 to 7.0 mm, 0.85xe2x89xa6B/Axe2x89xa61.15, 0.85xe2x89xa6C/Bxe2x89xa61.15, and 0.7xe2x89xa6D/Cxe2x89xa61.0.
While the invention relates to a multi-piece solid golf ball comprising a solid core, an enclosure layer around the core, an intermediate layer of at least one layer around the enclosure layer, and a cover of at least one layer around the intermediate layer, the first requirement of the invention is that the enclosure layer and the intermediate layer each have a Shore D hardness of 10 to 50, and the Shore D hardnesses of the enclosure layer, intermediate layer and cover are selected to meet the relationship: the Shore D hardness of the enclosure layerxe2x89xa6the Shore D hardness of the intermediate layerxe2x89xa6the Shore D hardness of the cover. That is, the layers are made softer as they are positioned inside from the cover to the intermediate layer and then to the enclosure layer. This minimizes the energy loss by deformation upon impact and improves resilience. The second requirement is that the deflection or deformation A (mm) under an applied load of 100 kg of the solid core is 2.5 to 7.0 mm, the deflection B (mm) under an applied load of 100 kg of the first spherical body consisting of solid core+enclosure layer, the deflection C (mm) under an applied load of 100 kg of the second spherical body consisting of solid core+enclosure layer+intermediate layer, and the deflection D (mm) under an applied load of 100 kg of the ball are selected to satisfy the relationship: 0.85xe2x89xa6B/Axe2x89xa61.15, 0.85xe2x89xa6C/Bxe2x89xa61.15, and 0.7xe2x89xa6D/Cxe2x89xa61.0. That is, the change of rates of deflection under load of spherical bodies before and after the provision of a layer (that are represented by B/A, C/B and D/C) can be set approximate to 1. This minimizes the energy loss at the interface between adjacent layers upon impact and thus restrains the resilience from lowering. The third (optional) requirement is that the enclosure layer and the intermediate layer are formed mainly of thermoplastic resins of the same type. This improves the bond between the enclosure layer and the intermediate layer and contributes to resilience. These choices cooperate synergistically, achieving a multi-piece solid golf ball which has significantly improved flight performance and gives a very soft and pleasant feel.
In the second aspect, the cover is formed of a cover stock comprising a thermoplastic resin as a base component and an inorganic filler. Due to the reinforcing effect of the filler, the ball is outstandingly improved in durability against consecutive strikes.